Sulfur dye is a group of dye containing sulfur. It contains sulfur bonds consisting of two or more sulfur atoms in its molecule. When in application, it can be reduced to leuco dye to dissolve in water and dye the fiber. The features of sulfur dye are short production process, low prices, good stability, convenient application and it is a popular dye which can be applied to the dyeing of fiber, such as cotton, linen, rayon, etc. However, sulfur dye is insoluble in water and when it is use do dye something it needs sodium sulfide or other reducing agents to reduce the dye into soluble leuco dye. Lecuo dye can be dyed on fibers because of its affinity to fibers, and then can be restored into its insoluble state after oxidation coloration so that it can fix on fiber. For ease of use, the common sulfur dyes are reacted with sodium sulfite or sodium hydrogen sulfite to produce thiosulfate of dye whose solubility is 150 g/l at 20° C. and can be used in continuous dyeing. However, it does not have affinity with the fibers and the dyeing liquor should be pad dyeing to the fiber and then treated with reducing solution to make the dye generate leuco dye with high affinity and can be attached to fibers. Then it should be watered and oxidation fixing the color. The feature of this dye is good levelness. However, it is not common to applied in the dyeing cellulose fibers, but often applied in paper dyeing. Introducing carboxyl group to sulfur dye structure to synthesize carboxylic acid type sulfur dye which can not only give good water solubility to the dye but also make the dye can be combined with fiber (cotton, wool, silk and leather) with the covalent bonds by making use of the reaction of carboxyl group and therefore gives sulfur dye good application features. As a result, the development of carboxylic acid type sulfur dye in recent years has caught people's attention.
U.S. Pat. No. 5,383,961 use common sulfur leuco dyes to react with chloroacetic acid in aqueous medium to prepare carboxylic acid type water-soluble sulfur dyes with good water solubility and the contents of carboxyl groups are between 5 to 20%. The salt-free dyes synthesized can be obtained with precipitating in acid, filtering, washing with water, and the salt can also be removed with membrane separation. Professor ZHANG Shufen from Dalian University of Technology uses Sulphur Black to react with chloroacetic acid to obtain carboxylic acid type water-soluble sulfur dyes and the carboxyl group content of dye reaches 11.9% (Chinese Chemical Letters, 2005, 16 (4): 554-556; Coloration Technology, 2007, 123 (3):191-196).
Water-soluble sulfur dye containing carboxyl group is a research direction of the improvement and gentrification of sulfur dye. Currently, the only reported carboxylic acid type sulfur dyes are generated by the reaction of sulfur dyes with α-chlorinated carboxylic acid. There is no research on carboxylic acid water-soluble sulfur dyes generated by the reaction of sulfur dyes with other active compounds containing carboxylic acid to introduce the carboxyl group into the dye.